1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine, or the like, that performs image formation e.g. by electrophotography.
2. Description of the Related Art
In printers, copying machines, and the like, that perform image formation e.g. by electrophotography, there has been known calibration for adjusting image quality to desired characteristics by reading an image pattern formed on a recording material and performing density correction and gradation correction. For example, in Japanese Patent Laid-Open Publication No. S62-296669 and Japanese Patent Laid-Open Publication No. S63-185279, as a method of calibrating an output image from an image forming apparatus, there has been proposed a technique of reading image information, such as density, chromaticity, etc. of a specific pattern formed on a recording material, and then feeding back the image information to image forming conditions, for improvement of image quality.
Further, in Japanese Patent Laid-Open Publication No. H07-261479, there has been proposed image calibration in which a latent image potential and a developing contrast potential are controlled by a charge bias voltage and a developing bias voltage in order to correct maximum image density, and a gradation correction table is changed in order to correct gradation characteristics.
In the above-mentioned Japanese Patent Laid-Open Publication No. S62-296669, Japanese Patent Laid-Open Publication No. S63-185279, and Japanese Patent Laid-Open Publication No. H07-261479, although gradation correction is performed using a pattern for gradation correction, which is formed on a recording material, this requires a user to place the recording material on which the pattern is formed on an original platen glass, and cause an image reader unit to read the pattern, which costs the user much time and labor. To eliminate this inconvenience, for example, there has also been proposed and realized a method of performing correction by optically detecting an amount of toner of a toner image formed on an image bearing member, such as a photosensitive member, an intermediate transfer member, or the like.
However, a detection unit which optically measures an amount of toner has a characteristic that if a surface of an image bearing member, such as a photosensitive member, an intermediate transfer member, or the like, is covered with a predetermined amount of toner, even when a larger amount of toner than the predetermined amount is adhered to the surface, a difference in the amount of toner is difficult to be detected. For example, “a” in FIG. 16A illustrates a state where the surface of the image bearing member is covered with a single layer of toner, with uncovered areas therein. “b” in FIG. 16A illustrates a state where the surface of the image bearing member is uniformly covered with a single layer of toner. “c” in FIG. 16A illustrates a state where the surface of the image bearing member is covered with two layers of toner.
FIG. 16B is a graph of an output from the optical detection unit with respect to an adhering toner amount, as obtained when toner on the surface of the image bearing member in the respective states of the above-mentioned “a”, “b”, and “c” is measured by the optical detection unit, in which the horizontal axis represents the adhering toner amount and the vertical axis represents the output from the optical detection unit.
According to the graph in FIG. 16B, although there is a large difference in the output between the states of “a” and “b”, there is little difference in the output between the states of “b” and “c”. For this reason, conventionally, in a higher density range than a range in which a difference in the output is generated (difference can be read), execution of gradation correction is limited to the range in which the difference can be approximated by a linear line or differences can be read. This causes a problem that in the gradation correction using the optical detection unit, the measurement in the high density range is low in accuracy or is impossible.